Final answer:
In hyperkalemia, the resting membrane potential of a muscle fiber becomes depolarized, as the increased extracellular concentration of potassium ions reduces the concentration gradient and leads to a more positive resting membrane potential.
Step-by-step explanation:
In hyperkalemia, the resting membrane potential of a muscle fiber becomes depolarized relative to its normal value. The resting membrane potential is the electric potential difference across the plasma membrane, typically at -70 mV, with the inside of the cell being more negative compared to the extracellular fluid. This potential is predominantly determined by the concentration gradients of sodium (Na+) and potassium (K+) ions across the cell membrane. The sodium-potassium ATPase actively pumps K+ ions into the cell and Na+ ions out, while K+ ions also leave the cell through potassium channels more freely than Na+ enters, contributing to the negative charge inside the cell.
During hyperkalemia, the extracellular concentration of potassium ions (K+) is elevated, which reduces the concentration gradient for potassium to leave the cell freely. With less K+ leaving the cell due to a weakened concentration gradient, the resting membrane potential becomes more positive or depolarized, affecting the cell's excitability and potentially leading to disturbances in muscle function, including that of the heart.